I too was puzzled by this, after trying a Search for Benchmarks near my home location, to be returned a blank search - as I have visited hundreds near me (and >5000 in the UK, not counting Trigs). Benchmarks (with Flush Brackets) aren't only on trigs, but also on houses, bridges, churches etc: see www.bench-marks.org.uk for a more or less complete list of Flush Brackets in the OSGB area (which even includes a couple in Gibraltar!). These too are loggable. So are Cut BMs for that matter!
The idea of getting a few armchair ticks via my TrigpointingUK profile is tempting - if I ever figure out this greasemonkey stuff